So there is the fact as to why I do not do this with project files and different work spaces.
The project file does not drive the Vault Add-in for what Vault to log in to.
Much like all other parts of the ADSK softwares it tends to remember the last place you went or last settings that were used.
So I have several Vaults due to access restrictions for DOD work.
Here is what I have done.
I first off have my older overall Vault that is setup using a single workspace and single project file through all of the users.
This Vault also contains all of the library, template, and design standard directories at the root. All of these types of files are checked out in the main Vault to a default users named "Library" on my machine. Then all of those types of files are then "gotten" to each Vault connected workstation. Since they are checked out on my machine they are protected (I know the fact that they are libraries protects them also somewhat).
Here is what that looks like:

Now I have several other Vaults on my ADMS but everyone still uses the same project file. What that means is all of the Library type of content can still be controlled in the main Vault but is still available locally because the structure is the same.
The user just has to change the Vault they are connected to through the IV Add-in and off they go to work.
They do need to make sure they are connected to the correct Vault but aLL else is the same.
As a matter of fact the top level Working Folder is also still the same. The difference in folder structure happens below that level so one of the other Vault just looks like this at the root.

You will note that this other Vault still also has a Library folder for my iPart factories and Content Center folder.
This is because these types of files are inserted into your design and even though they are "Library" in nature they still must travel up and down into the Vault with your designs. So they must exist in the Vault you are talking to also.
So with this method the user only needs to change which Vault they are connected to. And since it is the same user account in the ADMS all they have to do in the IV add-in is log out, use the drop down to change Vaults, and log back in. If they had previously select log in auto next session the dialog box remembers their PW.